1. Field of the Invention
The present invention relates to a structure of a pixel portion in a display device.
2. Description of the Related Art
In recent years, a display device which is thin and lightweight as compared to conventional cathode-ray tube display devices, a so-called flat panel display, has been developed. As a typical example of the flat panel display, a liquid crystal display device is known. In addition, as a new flat panel display, a display device with the utilization of an electroluminescence element (EL element) has been developed.
In a liquid crystal display device, liquid crystals are sealed between two sheets of transparent substrates, orientation of liquid crystal molecules is controlled by application of voltage to change light transmittance, and a predetermined image or the like is optically displayed. Since liquid crystals do not emit light by itself, a backlight unit serving as a light source is provided on the back of a liquid crystal display panel in a liquid crystal display device.
In addition, an EL element has a structure in which an electroluminescence material is sandwiched between a pair of electrodes. The EL element emits light by application of voltage between the pair of electrodes. A pixel is formed with this EL element, whereby a display device can be formed.
In either case, an electrode is necessary in order to form a pixel of a flat panel display. A pixel electrode needs to transmit light; therefore, a light-transmitting conductive film is used. Indium tin oxide (ITO) is known as a typical light-transmitting conductive film.
Indium, which is to be a main material of the light-transmitting conductive film, is a by-product generated in a slight amount in a process of refining zinc, and the production amount is slight worldwide. Therefore, as the production amount of the flat panel display increases, the scarcity value increases; thus, the cost rises. High cost of indium not only constitutes a factor of the high production cost but also limits the production amount of the flat panel display if the demand of the market cannot be filled. Specifically, the supply shortage of indium constitutes a factor in disturbing industry development. Therefore, a light-transmitting conductive film using hafnium is known as a substitution, for example (see Patent Document 1: Japanese Published Patent Application No. 2003-59343). However, hafnium, which is a rare metal, has a few reserves; thus, instability of the supply is undeniable.